1. Field of the Invention
The present invention generally relates to apparatuses for detecting end of battery life, and more particularly, to an apparatus for detecting end of battery life and informing users thereof used in a portable equipment powered by a battery, such as watch, calculator or so-called electronic organizer.
2. Description of the Related Art
Referring to FIG. 1, a conventional apparatus for detecting end of battery life includes a voltage detecting circuit 1 connected, in parallel with a main system 12, to a battery 2 to get, together with the main system 12, power supply from the battery 2 for detecting that voltage of the battery 2 has fallen below a predetermined value and informing users of an end of battery life. The battery 2 has its negative electrode grounded.
The voltage detecting circuit 1 includes an input terminal 4 connected to the positive electrode of the battery 2 through an interconnection line, l1, two resistances 6 and 7 connected in series and in the stated order between the input terminal 4 and ground, a reference voltage supply circuit 9 connected to the input terminal 4 for supplying a predetermined reference voltage, and a comparator 10 having a non-inverted input terminal connected to a connection node 8 of the resistances 6 and 7 and an inverted input terminal connected to output of the reference voltage supply circuit 9 and getting power supply for operation from the battery 2 through the input terminal 4. The main system 12 includes a display unit 3, to which output of the comparator 10 is coupled through an output terminal 5.
The conventional apparatus operates as follows. The input terminal 4 receives voltage Vcc from the battery 2. The voltage Vcc is divided by the resistances 6 and 7 and a resulting voltage V.sub.DET at the connection node 8 of the resistances 6 and 7 is supplied to the non-inverted input terminal of the comparator 10. The voltage V.sub.DET varies with the fluctuation of the voltage Vcc supplied to the input terminal 4 from the battery 2.
The reference voltage supply circuit 9 receives the voltage Vcc from the input terminal 4, outputs a predetermined certain reference voltage V.sub.REF and applies the same to the inverted input terminal of the comparator 10.
The comparator 10 compares the voltage V.sub.DET WITH the reference voltage V.sub.REF. If a relation V.sub.DET .gtoreq.V.sub.REF stands, the comparator 10 applies a low-level signal DET to the display unit 3. If another relation V.sub.DET &lt;V.sub.REF stands, the comparator 10 applies a high-level signal DET to the display unit 3.
The display unit 3 includes a buzzer, a liquid crystal display unit or the like incorporated in the main system 12. The display unit 3 does nothing when the output signal DET of the comparator 10 is at the low level. When the signal DET applied from the comparator 10 is at the high level, the display unit 3 sounds the buzzer or displays with the liquid crystal display unit a warning that the life of the battery 2 is near its end. Operators can, therefore, prevent voltage of the battery 2 from falling the below a predetermined value by replacing the same with a new one.
Referring to FIG. 2, voltage of the battery 2 gradually decreases with the lapse of operating time thereof. A point at which voltage of the battery 2 becomes smaller than a predetermined voltage value V0 is defined as the life end of the battery 2. By setting values of the resistances 6 and 7 properly, therefore, the reference voltage V.sub.REF can be selected to be substantially equal to the voltage V.sub.DET at the connection node 8 of the resistances 6 and 7 when voltage of the battery 2 is V0. When the voltage Vcc of the battery 2 falls below the voltage V0, or at the time t0, life of the battery 2 is regarded as having come to an end.
As described above, the supply voltage applied to the main system 12 can be prevented from falling below the reference value by constantly monitoring voltage of the battery 2 with the voltage detecting circuit 1. However, such a conventional apparatus for detecting end of battery life has disadvantages as described below. Referring to FIG. 1, current flows constantly from the battery 2 to ground through the resistances 6 and 7. Further, the comparator 10 consumes power supplied from the battery 2 on a constant basis. Therefore, life of the battery 2 is shortened due to the existence of the voltage detecting circuit 1. When the power consumption of the main system 10 is relatively small, life of the battery 2 may be unduly shortened due to the power consumption of the voltage detecting circuit 1.